Field based spectral radiometer

ABSTRACT

A robust two spectral band radiometer for long-term stand-alone spectral radiance measurements in the field is provided. The instrument can be used to monitor various surface parameters over prolonged periods of time by automatically collecting spectral radiance measurements at a user selected time interval (minutes to days). Two main applications are the monitoring of water surface parameters, such as total SSC and turbidity, and on-land vegetation by collecting spectral radiance measurements in a broad visible red and near-infrared spectral bands. Use for other application is possible using different spectral bands and multiple radiometers. Also included is the use of a ratioing technique to correlate the spectral radiance values rather than spectral reflectance values to the surface parameters of interest; this simplifies both the filed instrumentation requirements and post processing procedures.

FIELD OF THE INVENTION

[0001] This invention relates to a spectral radiometer designed to beleft in the field on a stand-alone basis for prolonged periods of time(months to years) to measure the spectral characteristics of variousearth surface targets in two bands.

BACKGROUND OF THE INVENTION

[0002] For several decades, spectral radiance data have been used tohelp map and monitor the earth's surface. The advent of civiliansatellite imaging systems in the early 1970s propelled this technologyinto widespread use both in image format and in situ field measurements.Digital imaging systems carried on-board earth-orbiting satellitescollect images using optical systems that record the earth surfacespectral characteristics in various bands (e.g., the brightness/color invisible and near-infrared (NIR) spectral bands). Current satelliteimaging systems have expanded spectral band coverage compared to thoseused in the 1970s and early 80s (e.g., short-wave IR—SWIR), resulting inthe need for more sophisticated field instruments with increasedspectral measurement capabilities. In order to help identify the designof future satellite imaging sensors and to collect spectral radiancedata in the field for use with current state-of-the-art satellite andairborne imaging systems, the need to design and build more complex andsophisticated spectral radiometers for in-the-field use has driven boththe cost and constraints of such an instrument quite high. The cost ofspectral radiometers now range from $12K to $80K for field instrumentsthat can collect data up to 1024 spectral bands. For applications thatrequire only a few bands (e.g., 2 to 6), with a high temporal resolutionover a prolonged period of time, using a current state-of-the-artspectral radiometer is out of the question. Both in cost and design thecurrently available spectral radiometers are not made for long-termstand-alone field use. Therefore, they are used only for short-termstudies and applications while the user is in the field, meaning thatthey cannot provide the high temporal resolution needed for long-termstudies and operational monitoring of the earth's surface.

[0003] Spectral radiometers were developed for in-the-field use forremote sensing in the early 1970s. Radiometers with four broad spectralbands were designed first, then the number of spectral bands began toincrease and the band width to decrease to allow better total spectralmeasurements to be collected of the cover types of interest (i.e.,soils, vegetation, or water). In the late 1980s and early 1990s,spectral radiometers having over 250 narrow spectral bands were designedand built and now ones with 512 to 1024 bands (hyper-spectral) arebecoming the standard. However, there are a number of problems with theuse of current hyper-spectral radiometers, including:

[0004] 1) The amount of data they are designed to collect is typicallyan “overkill” for many applications and operational monitoring of theearth's surface (e.g., surface waters and general vegetation cover).That is, over 500 bands of spectral radiance data are not needed formany applications, especially for non-research operational andmonitoring uses.

[0005] 2) The cost for the much more complex and sophisticatedinstruments now on the market ranges from $12,000 to $80,000. This costis quite high for most non-research applications, especially if severalto tens of them are needed for good spatial monitoring over a regionalarea.

[0006] 3) The spectral radiometers currently on the market are designedto be used in the field for a relatively short amount of time by aperson while doing fieldwork. Current radiometers are not designed to beleft in the field for long periods of time (i.e, months to years) in astand-alone node to automatically collect spectral radiance data with ahigh temporal resolution.

[0007] 4) The current spectral radiometers with over 500 bands havenarrow bandwidth, so the overall signal-to-noise ratio is low comparedto the more broadband width radiometers. Therefore, for low radiancetargets, such as surface waters, the noise levels will typically behigher than for those collected by a less complex broadband radiometer.

[0008] 5) A spectral radiometer with four bands was developed in theearly 1970s by Exotech, Inc., for use in the field while the operatorwas present; it is not designed to be left in the field on a stand-alonebasis for prolong periods of time.

[0009] Satellite image data can be used to monitor various features onthe earth's surface with additional spectral windows. However, majorproblems with using satellite image data to monitor surface waterparameters, and on-land vegetation cover, are that the combination oftemporal and spatial resolutions often needed are well beyond thecapability of current satellite imaging systems. To obtain the temporalresolution needed of minutes to hours, and a spatial resolution of oneto three meters required to see the surface waters of rivers, or fordaily monitoring of vegetation within a small area, a field basedinstrument is required.

[0010] A number of other types of radiometers have been disclosed whichhave been used for a variety of purposes.

[0011] Goetz et al., in U.S. Pat. No. 4,345,840, disclose a hand held,self-contained dual beam rationing radiometer for identifying selectedmaterials that reflect radiation within a predetermined band, preferablyin the IR or visible range. The apparatus includes two pivoting opticaltrains directed toward the same target. Each train has a separate filterfor selection of the narrow spectral bands to be ratioed, by means of adividing circuit, to identify a particular substance based on its knownspectral characteristics.

[0012] Spiering et al., in U.S. Pat. No. 6,020,587, disclose a devicefor measuring plant chlorophyll content by collecting light reflectedfrom a target plant. A beam splitter separates the light into distinctwavelength bands or channels. Photo-detectors and amplifiers within thedevice then process the bands, converting them into electrical signals.

[0013] Gupta discloses, in U.S. Pat. No. 4,996,430, a device fordistinguishing target objects having substantially identical reflectanceratios for two separated wavelengths (lambda-1, lambda-2), frombackground objects possessing different reflectance values for the sametwo wavelengths. This device includes an active optical sensor withfirst and second transmitters that transmit signals at wavelengths oflambda-1 and lambda-2, respectively. When the transmitted signalsreflect off of an object, a receiver senses the reflected signals, whichare then processed through a high-speed preamplifier and amplifier,producing voltages V1 and V2 at the receiver's output. A conventionalratio calculating circuit then calculates a ratio of V1 and V2, which isthen compared to a predetermined threshold value.

[0014] Levin et al., in U.S. Pat. No. 6,031,233, disclose a handheldspectrometer for identifying samples based upon their IR reflectance.The device includes a window and adjacent optical bench. The optics,which align directly with the sample under investigation, consist of abroad band IR light shining onto an acousto-optic tunable filtercrystal, the latter passing narrow band IR light with a swept frequency;a lens focusing the IR light through the window onto the sample; and areflectance detector aligned with the housing window to detect lightreflected from the sample. A computer mounted in the housing comparesthe reflectance spectrum with stored data and identifies the samplematerial.

[0015] Novinson, in U.S. Pat. No. 4,527,062, discloses a portable IRspectrophotometer for testing samples of organic construction materials.A single source of IR light is divided into two beams. The first beam isdirected to the surface of the target sample, from which the light isreflected at least four times. The second, reference, beam is directedtoward a neutral surface. The two beams are then combined and focusedonto a detector element. The detector output is proportional to theenergy absorbed by the test sample. A pen recorder attached to theapparatus generates a graphic “fingerprint” of the sample's reflectancespectrum.

[0016] Typically, spectral bands used to study and monitor waterparameters fall within the blue and green spectral bands, not red andnear IR as to be used in this application.

[0017] Traditionally, spectral radiance measurements must be convertedto surface reflectance, requiring a more complicated setup andadditional instrument capabilities in the form of a calibrator or asolar irradiance measurement. By ratioing the spectral radiances of twobands, as to be done in this design, the need to collect measurements ofeither a calibrator or solar irradiance at the same time the targetsurface readings are collected is not needed.

SUMMARY OF THE INVENTION

[0018] A main objective of the present invention is to overcome thedeficiencies in the existing instruments and to provide a robust,dual-band spectral radiometer suited for long-term stand-alone fielduse. By using the ratio of spectral radiance values to correlate to theparameters of interest, rather than spectral reflectance values as istypically used, it eliminates the need for either calibrator or solarirradiance measurements, thereby reducing the cost and complicationencountered when these extra measurements are needed. Two immediateapplications of this design will be to measure and monitor the spectralcharacteristics of water surfaces, which can be correlated to waterparameters such as total suspended sediment concentration (total SSC)and turbidity, and on-land vegetation spectral signatures, which can becorrelated to vegetation cover/density, with possible correlation tovegetation health/water stress.

[0019] This invention includes a method to effect water surface andon-land vegetation long-term stand-alone monitoring by utilizing thesame two spectral bands. For these applications the two spectral bandshave a broadband width and cover the visible red and near-IR portions ofthe spectrum. However, the spectral radiometer's general design is suchthat for other applications that will arise a different set/pair of twobands can be used (e.g., blue and red, and/or narrow instead of broadband widths). Also, if it is determined that some applications requiredmore than two bands (e.g., five or six), then sets of this radiometercan be used (2 bands per radiometer times 3 radiometers will allow a sixband set up, if needed).

[0020] Again, another main objective of the invention or method is toprovide a means to correlate spectral data collected in the field withthis instrument to surface parameters of interest, such as total SSC orvegetation density, without the need to also collect spectralmeasurements of a calibrator or solar irradiance at the same time.

[0021] This invention includes a new procedure that uses the ratio ofmeasured spectral radiances in two bands. In the case of surface watersand on-land vegetation it uses broad bands in the visible red andnear-IR portions of the spectrum, for long-term operational monitoringof the parameters of interest. The ratio of the spectral radiances intwo bands has a direct and high correlation to the ratio of reflectancevalues, therefore, this can often serve as a replacement to using actualsurface reflectance in the correlation of the spectral measurements tothe surface parameter of interest. This means the need for either acalibrator or solar irradiance measurements to be collected at the sametime is eliminated, thus making the procedure both less expensive andless complex than procedures that use reflectance values. It should benoted that if either a calibrator or solar irradiance measurements areneeded, the maintenance requirements of a long-term stand-alone set upwill dramatically increase. This will be because the calibrator or thesky looking optics will need to be kept clean so the measurements arereliable and comparable from day-to-day.

[0022] The instrument in the present invention is designed to collectspectral radiance measurements in two bands. For the surface waters andon-land vegetation monitoring applications it will use broad-width bandsin the visible red and near-infrared portions of the spectrum. Thespectral radiance values from these two broad-width bands have a goodcorrelation with several surface water parameters, including totalsuspended sediment concentration (total SSC) related to silt and clay aswell as turbidity, on both inland and coastal waters. The spectralradiance values from these same two spectral bands also have a goodcorrelation with vegetation cover or density, and have been shown tooften correlate with vegetation maturity and, in some cases, vegetationhealth—water stress.

[0023] Two immediate applications of the instrument are to measure thespectral characteristics of water surfaces or on-land vegetation to mapand monitor the total suspended sediment concentrations and turbidity ofwater or vegetation cover/density.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 shows the correlation between silt/clay and irradiance.

[0025]FIG. 2 shows total SSC collected over a two month period.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The instrument of this invention is a simple but robust twospectral band radiometer that can be used in the field on a long-term(months to years) stand-alone basis to automatically collect hightemporal resolution data directly related to the spectral radiancecharacteristics of the target/surface it is set up to monitor. The datawill be useful for both operational monitoring to detect change, as wellas for research studies that require this type of information. Twoimmediate applications for this instrument and the spectral radiancepost-processing procedure included in the invention are the monitoringof surface in-land waters for total suspended sediment concentrationsand turbidity, as well as the on-land vegetation cover. The samplingtemporal resolution can be selected by the user and will typically bedepended on the application (e.g., every 15 to 30 minutes for surfacewater monitoring and once per day or week for on-land vegetationmonitoring). For applications needing more than two spectral bands, butstill on the order of fewer than ten, multiple instruments can be set upwith a different pair of spectral bands for each radiometer.

[0027] The instrument of this invention can be installed either as asingle unit or in groups in the field for prolonged periods of times(i.e., months to years), to collect spectral radiance measurements on arelatively high temporal frequency/resolution. The temporal resolutionneeded will depend on the application and can be selected by theindividual user. The temporal resolution can range from a measurementcollected every few minutes to every few days (e.g., every 15 minutesfor surface water monitoring to once per day or per week for monitoringon-land vegetation). The direct output of the instrument is voltages forthe two bands that are post-processed to convert to spectral radiancevalues. The spectral radiance values are then ratioed to effect a firstorder removal of the effect of both sun elevation and cloud covervariations encountered throughout the day and year. After the initialcorrelation of the resulting ratio values to the parameter of interest,for example water sample analyses results or field vegetation transectinformation, new values can be used to monitor the surface parameters ofinterest (total SSC and/or turbidity in the case of surface waters orchanges in vegetation spectral signature in on-land setting).

[0028] The two bands that will be used for both the surface water andon-land vegetation monitoring, that is broad-width bands in the visiblered and near-infrared portions of the spectrum, are widely used by theremote sensing community to study and map vegetation, but theirapplication to surface water monitoring is typically not used and is anew application of this invention (typically the blue and green portionsof the spectrum are used). Another major difference in the procedure ofthe present invention is the use of spectral radiance rather thanspectral reflectance values for analysis and monitoring. Since spectralradiance measurements in the field are collected under various sunelevations and shading conditions, existing procedures convert the datato spectral reflectance values to normalize for the variable lightingconditions before using them in analysis and monitoring applications(i.e., spectral reflectance values are, to a first order, independent oflighting conditions). However, a disadvantage of using spectralreflectance values is that a second set of spectral radiancemeasurements must be made at the same time that target measurements aremade. Measurements of either a bright standard calibrator or solarirradiance in the bands being used must be made at the same time toallow the radiance values to be converted into spectral reflectancevalues. This makes the collection of the data in the field morecomplicated and increases the expense and complexity of the instrumentneeded. In fact, besides increasing the complexity in collecting thedata for long-term stand-alone data collection requirements, thisbecomes a major problem and could make the collection of data over along-term basis not reasonable. The reason for this is the requirementto keep the calibrator or the sky ward sensing optics clean so thatreadings are not affected by dirty surfaces; this can become a majormaintenance problem, especially when the radiometer is set-up in remoteareas.

[0029] The procedure of the present invention circumvents this problemby using the ratio of the spectral radiance values (i.e., correlate theratio of two bands to the surface parameter of interest, rather than theindividual band spectral values). It turns out the ratio of spectralradiance values is highly correlated to the ratio of the spectralreflectance values (this is shown in the equations below), therefore,rather than developing a model that correlates spectral reflectancevalues to the parameter of interest (as is usually done), one can usethe ratio of spectral radiance values. By using the ratio of thespectral radiance values when correlating to the desired surfaceparameters (e.g., total SSC, turbidity, or vegetation density), there isno need to convert to surface reflectance values, thereby eliminatingthe need for either a calibrator or sky solar irradiance measurements.The use of ratio spectral radiance values of two bands close to eachother, such as in the present invention, automatically eliminates, to afirst order, most problems associated with changing sun/shade conditionsencountered throughout the day or the year, and the resulting spectralradiance ratio is highly correlated to spectral reflectance ratiovalues.

[0030] Specifically, provided below (see equation 1) is a generalequation widely used to convert spectral radiance to spectralreflectance values. Also shown, is the novel equation (equation 3) usedin the procedure of the present invention to demonstrate that thecorrelation between the ratio of spectral reflectance and the ratio ofspectral radiance values are highly correlated, therefore, either onecan be used when a relationship exists between one of them and a surfaceparameter of interest (e.g., total SSC, turbidity, or vegetationdensity). This is important because spectral radiances can be measuredusing simplified techniques compared to spectral reflectances.

Reflectance=[Π*D*D*(Rad−HazeRad)]/(E _(o)*CosN)  (1)

[0031] where

[0032] Reflectance=Surface reflectance in the given spectral band

[0033] Π=3.14259 (pi)

[0034] D=Sun-Earth distance in AU (approx. 0.98 to 1.02)

[0035] Rad=Radiance in the given spectral band (radiance is equal toat-satellite values if dealing with satellite borne images, however,with ground based readings, such as in our case, it is equal to theactual surface/target radiance)

[0036] HazeRad=Atmospheric haze radiance; equal to zero for field or onthe ground readings

[0037] N=Solar zenith angle, measured in degrees (90 minus sun elevationangle)

[0038] E_(o)=Total solar irradiance in the given spectral band

[0039] To convert spectral radiance data collected in the field/on theground to spectral reflectance values, spectral radiance measurementsmust also be collected at the same time in the same bands of either abright calibrator that has a known reflectance or incoming solarirradiance (Eo in equation 1) in the bands being used. The calibrator orincoming solar irradiance measurements gives the information required tocovert to reflectance values, thereby, correcting for differences inlighting conditions (i.e., sunny or shaded conditions, as well asdifferences in sun elevation angles throughout the day or year), as wellas the earth-sun differences (Eo and D terms in equation 1). In order toeliminate the need for the extra measurements, which requires either asecond radiometer if a calibrator option is used or a more complexdesign if only a single radiometer is used, a new procedure has beendeveloped that uses the ratio of the spectral radiances of the two bandsinstead to develop the model/relationship needed for the parameters ofinterest. Keep in mind, that for long-term stand-alone stations thecalibrator or sky ward pointing optics might not be an option because ofthe question of keeping the surfaces clean over a period of weeks tomonths between field visit. Solving equation (1) for in-the-fieldradiance gives:

Rad=(Reflectance*Eo*CosN)/(pi*D*D)  (2)

[0040] The HazeRad term in equation 1 is equal to zero on the groundbecause of the lack of atmospheric haze effects compared to imaging fromspace, so it is dropped and not present in equation 2. Using equation 2,the radiance of two spectral bands, for example the red and near-IRbands, collected simultaneously by the radiometer can be put into theradiance radio algorithm as follows: $\begin{matrix}{\frac{{Rad}_{red}}{{Rad}_{{near}\text{-}{IR}}} = \frac{{Reflectance}_{red}*E_{ored}*{Cos}\quad {N/\left( {\pi*D*D} \right)}}{{Reflectance}_{{near}\text{-}{IR}}*E_{{onear}\text{-}{IR}}*{Cos}\quad {N/\left( {\pi*D*D} \right)}}} & (3)\end{matrix}$

[0041] The CosN, Π, and D terms cancel, so the effect of sun elevation(CosN) and earth-sun distance (D) they represent is automaticallyeliminated. The radiance ratio is thus reduced to equation (4), below:$\begin{matrix}{\frac{{Rad}_{red}}{{Rad}_{{near}\text{-}{IR}}} = \frac{{Reflectance}_{red}*E_{o{({red})}}}{{Reflectance}_{{near}\text{-}{IR}}*E_{o{({{near}\text{-}{IR}})}}}} & (4)\end{matrix}$

[0042] The values E_(o(red)) and E_(o(near-IR)) increase and decreasetogether in accordance with variations in sun elevation and shadingconditions, which is influenced by the both the time of day and year. Toa first order the value of the ratio of E_(o(red)) to E_(o(near-IR)) isapproximately constant. Thus, the ratio of spectral radiance values,which can be generated without the need for a calibrator or solarirradiance measurements, correlate directly with the ratio of spectralreflectance values. Therefore, a major advantage of using the ratio ofspectral radiance values to generate the model/relationship required,rather than spectral reflectance values, is that neither a calibrator orsolar irradiance measurements are needed—this reduces the design andcomplexity requirements of the field instrument and its set up byremoving the need to keep the surface of either a calibrator or skyviewing optics clean over a prolonged period of time. Because thespectral radiance ratio is highly correlated to the spectral reflectanceratio of the target of interest (e.g., surface water or on-landvegetation), the spectral radiance ratio shown in equation (4) is usedin this procedure in place of reflectance values that require acalibrator or sky viewing optics. Consequently, the present inventioneliminates the need for more complicated instrument requirements andset-up traditionally needed to monitor surface parameters of interest.The present invention makes available a procedure that can be used inthe field to monitor changes in various surface spectral characteristicsfor prolonged periods of time without the need for frequent maintenancerequirements (to keep the calibrator or skyward pointing optics clean).In two immediate applications dealing with surface waters and on-landvegetation broad bands in the red and near-infrared portions of thespectrum are used. However, in other applications two other bands can beused instead, depending on the application. Also, if more than two bandsare needed to appropriately monitor the spectral properties of asurface, a combination of two to several of the new radiometers can beused. For example, if a user wants to monitor the atmosphere forhazy/pollution changes a radiometer with two bands in the lower blue andlower green portions of the spectrum could be used (or blue and redspectral bands), and they can be more narrow than the ones being usedfor the above two immediate applications. If it is found that 3 to 5more narrow bands are needed to monitor the parameter of interest, thentwo or three radiometers, with two narrower bands per radiometer can beused in the field set up.

[0043] Again, the use of this new robust spectral radiometer and theratio of spectral radiances (rather than reflectance as is common) tohelp map and monitor various earth surface parameters has the followingadvantages: designed for long-term stand-alone field use, collectsspectral measurements having a relatively high temporalfrequency/resolution (e.g., every 15 min. to twice per day to once perweek), low cost compared to existing instruments, option to use as fewas only two spectral bands with the option of setting up more than oneinstrument for applications needing more than two bands, data can becorrelated to satellite images, plus the spectral data can be correlatedto the parameters of interest without the need to use a calibrator ormeasure incoming solar irradiance because it works with the ratio of thespectral radiance values rather than spectral reflectance.

[0044] An Exotech four band spectral radiometer was used to collectspectral radiance measurements of the surface waters on the ColoradoRiver at the bottom of the Grand Canyon. This radiometer was designedand developed in the early 1970s and contains broad blue, green, red,and near-infrared bands. The radiometer was protected from rain anddirect sunlight in an attempt to design an initial version of astand-alone long-term instrument. The spectral radiance data collectedusing this radiometer set up, which was not designed for long-term standalone use, were correlated to total suspended sediment, silt/clay, andsand concentrations (mg/l) derived from water samples collected duringthe spectral radiance measurements. The correlation between the spectralmeasurements and both total SSC and silt/clay of the water samples wasvery good with an R value of 0.95, while the correlation with sand wasnot nearly as good with an R value of only about 0.60. FIG. 1 shows therelationship between silt/clay and radiance ratio (basically identicalto total SSC); the model derived for this particular data set to relatespectral radiance values to silt/clay concentrations is:

Silt/clay=9.8957*exp(3.5*spectral radiance ratio)

[0045] This model was used to predict the silt/clay (and total SSC) formeasurements collected during the time water sample results were notcollected. FIG. 2 is a graph showing the total SSC for the time periodcovering September to October 1999. This has been compared to the sametime period in 2000 and a signature difference due to monsoon raindifferences between the two years is easily detected.

[0046] Operational monitoring of total SSC and silt/clay with this levelof temporal resolutions and length of time are not typical, and to do itwith an instrument that physically does not touch the water, whichdramatically reduces the maintenance requirements, has not previouslybeen done. This is an example of the potential of this instrument forhigh temporal resolution long-term automatic data collection for bothoperational monitoring and research applications.

[0047] The Exotech radiometer used in this experiment was designed andbuilt in the early 1970s, so in the design of this new simple, butrobust, radiometer we take advantage of the new technology that is nowavailable. For example, the design of the new radiometer is such that itcan be set up to either pass the voltages measured (which are directlyrelated to spectral radiances) directly to a data logger and have itstore the values offline, or make use of the new flash card technologyto store the values within the radiometer unit. The values can then bedownloaded at a later date onto external storage using a laptopcomputer. The option selected will depend on the set up; that is, willit be a unit that is the only one being used at that location (thiswould probably use the internal flash card option), or is this one ofseveral instruments set up at that location (this set up would probablyprefer the outside data storage option because a data logger is alreadyavailable). By having this option the extra cost of a data logger devicecan be eliminated when the instrument is the only one being used at thatlocation.

[0048] In the single instrument mode the instrument will also requirethe capability to have internally an analog-to-digital conversioncapability. The multi-instrument mode this conversion can be done by theexternal data logger.

[0049] The instruments will be equipped with the option of either manualor automatic gain settings. In the manual settings the gain for each ofthe two spectral bands is pre-selected and used at all times. In theautomatic mode if the light levels being measured are too low, the gainis automatically increased to get the reading into a desired range, thenthe selected gain values are used to collect the measurements at thattime; the gain values used are recorded for post processing purposes.The next time it makes a measurement the gains are again automaticallyoptimized for the brightness/radiance levels encountered at that time.

[0050] In the two immediate applications that the instrument will beused, that is to help monitor surface waters and on-land vegetation, thetwo spectral bands to be used correspond to those used to compute thewidely popular Normalized Difference Vegetation Index (NVDI) frommulti-spectral satellite and airborne image data. Therefore, theinstrument with this particular two band combination will give thosestudying and monitoring vegetation growth and density a tool to collectlong-term high temporal resolution in-the-field data automaticallywithout needing to be present during the data collection.

[0051] The foregoing description of the specific embodiments will sofully reveal the general nature of the invention that others can, byapplying current knowledge, readily modify and/or adapt for variousapplications such specific embodiments without undue experimentation andwithout departing from the generic concept. Therefore, such adaptationsand modifications should and are intended to be comprehended within themeaning and range of equivalents of the disclosed embodiments. It is tobe understood that the phraseology or terminology employed herein is forthe purpose of description and not of limitation. The means andmaterials for carrying our various disclosed functions make take avariety of alternative forms without departing from the invention. Thus,the expressions “means to . . . ” and “means for . . . ” as may be foundin the specification above and/or in the claims below, followed by afunctional statement, are intended to define and cover whateverstructural, physical, chemical, or electrical element or structureswhich may now or in the future exist for carrying out the recitedfunction, whether or not precisely equivalent to the embodiment orembodiments disclosed in the specification above; and it is intendedthat such expressions be given their broadest interpretation.

What is claimed is:
 1. A method for monitoring surface parameterscomprising collecting spectral measurements in two spectral bands;applying ratioing techniques to remove the effect of sun elevation andcloud cover variations; calibrating the resulting values to knownvalues; and monitoring the surface parameters of interest.
 2. The methodaccording to claim 1 wherein the surface parameters are selected fromthe group consisting of vegetation cover, vegetation density, andcombinations thereof.
 3. The method according to claim 1 wherein thesurface parameters are selected from the group consisting of suspendedsediment concentration in water, turbidity in water, and combinationsthereof.
 4. The method according to claim 1 wherein the spectral bandsmeasurements for the two immediate applications are visible red andnear-infrared.
 5. The method according to claim 1 wherein the ratioingtechniques comprising the following formula:$\frac{{{Radiance}({red})} = {{{Reflectance}({red})}*{{Eo}({red})}}}{{{Radiance}({nir})} = {{{Reflectance}({nir})}*{{Eo}({nir})}}}$

wherein Eo is the total solar Irradiance in a given spectral band isused in place of spectral reflectances.
 6. The method according to claim1 wherein the spectral measurements are collected at time intervalsranging from about 15 minutes to two weeks during daylight.
 7. A methodfor monitoring surface parameters comprising collecting as plurality ofspectral measurements in two spectral bands using a plurality of one toseveral radiometers covering different portions of the spectrum;applying ratioing techniques to remove the effect of sun elevation andcloud cover variations; calibrating the resulting values to knownvalues; and monitoring the surface parameters of interest.
 8. The methodaccording to claim 7 wherein the surface parameters are selected fromthe group consisting of vegetation cover, vegetation density, andcombinations thereof.
 9. The method according to claim 7 wherein thesurface parameters are selected from the group consisting of suspendedsediment concentration in water, turbidity in water, and combinationsthereof.
 10. The method according to claim 7 wherein the spectral bandsfor the two immediate applications are visible red and near-infrared.11. The method according to claim 7 wherein the ratioing techniquescomprising the following formula:$\frac{{{Radiance}({red})} = {{{Reflectance}({red})}*{{Eo}({red})}}}{{{Radiance}({nir})} = {{{Reflectance}({nir})}*{{Eo}({nir})}}}$

wherein Eo is the total solar irradiance in a given spectral band isused in place of spectral reflectances.
 12. The method according toclaim 7 wherein the spectral measurements are collected at timeintervals ranging from about 15 minutes to two weeks during daylight.